1. Technical Field
The present invention relates generally to differential current mode drivers and in particular to differential current mode drivers used to assert a conductor in a differential small computer system interface bus. Still more particularly, the present invention relates to a differential current mode driver which maintains nearly equal source and sink currents while providing a stable on-state (normal mode), a controlled on-to-tristate transition (power-down), and a fast tristate-to-on transition (power-up).
2. Description of the Related Art
One application of differential current mode drivers relates to the small computer system interface ("SCSI" or commonly called the "SCSI bus"). Various versions of the SCSI standard have been proposed, approved, and/or adopted. Specifically, a differential current mode driver may be employed in an emerging SCSI standard known as "SCSI Parallel Interconnect 2" or "SPI-2" as documented in a working draft bearing the same title, currently at revision 11 dated Sep. 10, 1996, which is incorporated herein by reference.
SCSI specifications allow for both "single-ended" and "differential" buses. "Single-ended" SCSI buses use an unbalanced electrical signal, or a single conductor for each bus signal with all conductors in the bus using a single, common-ground return. Such a single-ended bus is prone to picking up noise or interference, resulting in a severe limitation on single-ended SCSI bus cable lengths at higher signal frequencies due to ground bounce, cross-talk, signal attenuation and signal reflections. "Differential" SCSI buses use balanced or differential signals, with each conductor on the SCSI bus having its own return line isolated from the reference ground. As a result, much longer cables may be used with a differential SCSI bus.
A differential current mode driver may be used to assert a signal on a differential SCSI bus conductor. Ideally, the output source and sink currents of a differential current mode driver should remain nearly equal in magnitude and opposite in direction to have minimal disruption on the cable. This is true both when the signal is being actively asserted or actively negated as well as when the signal is entering or exiting the tristate condition. Furthermore, in transitioning between the on-state and tristate conditions, the output of a differential current mode driver to a differential SCSI bus conductor should not transition instantly or take an inordinate amount of time. Rather, a differential current mode driver driving a differential SCSI bus conductor should behave in a slew-rate limited fashion.
It would be desirable, therefore, for a differential current mode driver attached to a SCSI bus to maintain nearly equal source and sink currents while providing a stable on-state, a controlled on-to-tristate transition, and a fast tristate-to-on transition. It would further be desirable for a differential current mode driver driving a SCSI bus conductor to employ three distinct time constants to regulate normal operation, power-down, and power-up.